Transition from bifilar helix to waveguide for backward wave oscillator



Sept. 17, 1957 Filed April. 1. 1955 H. R. JOHNSON 2,806,975 TRANSITION mom BIFILAR HELIX T0 WAVEGUIDE FOR BACKWARD WAVE OSCILLATOR 2 Sheets-Shut 1 dllH r g -l HORACE RICHARD JOHNSON INVENTOR BY W W ATTORNEY Sept. 17, 1957 R. JOHNSON 2,306,975

TRANSITION mo IFILAR HELIX TO mwzcuws FOR BACKWARD WAVE OSCILLATOR Filed April 1, 1955 2 Shoots-Sheet 2 FIG. 2

HORACE RICHARD JOHNSON INVENTOR ATTORNEY United States Patent TRANSITION FROM BIFILAR HELIX T0 WAVE- GUIDE FOR BACKWARD WAVE OSCILLATOR Horace R. Johnson, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application April 1, 1955, Serial No. 498,656 6 Claims. c1. sis-3.6

This invention relates to microwave oscillators and more particularly to a backward wave oscillator employing an enlarged bifilar helix and incorporating structure for providing a transition between the bifilar helix and a waveguide over a broad range of frequencies.

As is generally known a backward-wave oscillator is a device wherein an electron stream is directed through a slow-wave structure at a velocity and in a manner to amplify an electromagnetic wave the energy of which is propagated by the structure in a direction opposite to that of the electron stream. An eletromagnetic wave of this type is typically designated as a backward wave. Bifilar helices are particularly adaptable for use as the slow-wave structure in this type of oscillator because of the high impedance they present to backward waves. This structure, however, presents the problem of providing a good tansition from the bifilar helix to a waveguide or other output structure over the frequency range of the oscillator. In the event that there is a poor transition to the output structure from the bifilar helix, it is generally realized that a large standing wave will result which causes variations in amplitude and frequency pulling in the output signal as the oscillator is tuned throughout its frequency range. Frequency pulling and amplitude variations are caused by reflections occuring at an extremity of the helix and having sufficient magnitude to produce internal feedback.

In accordance with the present invention a backwardwave oscillator employing a bifilar helix is provided having an improved transition therefrom to an output wave guide section with minimized variations in amplitude and frequency pulling throughout the operating frequency range of the oscillator. The apparatus providing the transition comprises two similar ferrules, each of which has an inside diameter substantially larger than the helix. These ferrules are disposed symmetrically with respect to the output extremity of the helix concentrically about its longitudinal axis. Each ferrule is connected to a different helix at diametrically opposite points. An output waveguide section symmetrically encloses the two ferrules so that the gap therebetween appears midway between the broad sides of the waveguide and at a distance of approxi-- mately one-quarter of a guide wavelength from a virtual or actual shorting plane within the waveguide. In operation, the electromagnetic wave propagated by the bifilar helix produces an electric field across the gap between the ferrules which in turn excites an output signal in the waveguide.

It is therefore an object of this invention to provide an improved backward-wave oscillator capable of producing an output signal with minimum amplitude variation and frequency pulling while being tuned over a wide range of frequencies.

Another object of this invention is to provide a backward-wave oscillator employing a bifilar helix with an improved transition structure to match the impedance of the helix to that of a waveguide.

Still another object of this invention is to provide a simple structure capable of etfecting a transition between a bifilar helix and a waveguide over a broad range of frequencies.

The novel features which are believed to be characteristic of the invention both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a cross-sectional view in partial elevation of an embodiment of the present invention together with associated circuitry; and

Fig. 2 is a section along line 2-2 of Fig. 1.

Referring now to the drawing, Figs. 1 and 2 show an embodiment of the present invention comprising an elongated cylindrical envelope 10 having an enlarged portion 11 at the left extremity as viewed in the drawing. Enlarged portion 11 houses an electron gun 12 for producing a hollow cylindrical electron stream. This elec tron stream is directed along a path that runs parallel with the longitudinal axis of envelope 10 by means of a solenoid 13. A collector electrode 14 is disposed at the extremity of envelope 10 farthest from electron gun 12 to intercept and collect the electron stream. Disposed contiguously about the path of the electron stream intermediate the electron gun 12 and collector electrode 14 is a bifilar helix 16. In accordance with the present invention a transition device 18 for matching the impedance of bifilar helix 16 to a rectangular output waveguide section 20 is disposed about the extremity of bifilar helix 16 nearest the electron gun 12.

More particularly, the electron gun 12 comprises an annular cathode 22 with a heater 24, a focusing electrode 26, and accelerating electrodes 28 and 29, the electrodes 26, 28, and 29 being provided with apertures in register with the electron emitting annular surface of cathode 22 to allow passage therethrough of the hollow cylindrical electron stream. Heater 24 is connected across a source of potential, such as a battery 30, one terminal of which may be connected to cathode 22 as shown. The electrode 26 provides an inner and an outer surface of revolution adjacent to and about the electron-emitting surface of cathode 22, as shown in the figure, to provide focusing of the electron stream. Cathode 22 and focusing electrode 26 are connected together and are, in turn, connected to an adjustable tap 32 of a potentiometer 34. Potentiometer 34 is connected across a source of potential 36, the positive terminal of which is connected to ground. The magnitude of the potential provided by the source 36 may be of the order of 5000 volts. Accelerating anode 28 is disposed in a plane normal to the longitudinal axis of envelope 10 to the right of focusing electrode 26 as viewed in the drawing. Anode 28 is maintained at a substantially fixed potential of the order of 200 volts positive with respect to ground by means of a connection therefrom to the positive terminal of a battery 38, the negative terminal of which is referenced to ground. Anode 29, on the other hand, is disposed intermediate the anode 28 and bifilar helix 16 in a plane normal to the longitudinal axis thereof and is maintained at ground potential by means of an appropriate connection thereto.

The solenoid 13 is positioned concentrically about the envelope 10 and is coextensive with the active length of the tube. An appropriate direct current is made to flow through the solenoid 13 by means of connections across a battery 40 to produce an axial magnetic field of the order of 600 gauss through the tube. The purpose of the magnetic field is to keep the electron stream ctinstrained in its path from the electron gun 12 to the collector 14. Also, inasmuch as a bifilar helix is employed as the slow-wave structure in the present invention, it is apparent that electrostatic focusing may be used in lieu of the magnetic field.

The bifilar helix 16, disposed about the path of the electron stream may have a circumference that is either large or small in comparison to a free space wavelength at the frequency of operation of the device such as, for example, 0.6 or 0.15 wavelength, respectively. Helices 42 and 44 constitute the bifilar helix 16. These helices 42, 44 have equal pitches and diameters and may be made of a material such as tungsten or molybdenum, the main prerequisite being that they retain their form, especially with respect to pitch and diameter. Also, the inner diameter of the hollow electron stream should be from 0.2 to 0.95 the inner diameter of the bifilar helix 16 in order that the electron stream may flow in the electric field of the wave propagated by the structure.

Bifilar helix 16 is maintained in position by three ceramic rods 46, 48 and 50 which have a length equal to that of the helix 16 and may, for example, be glazed at equal intervals about the outer periphery of helices 4 2, 44. Resistive coatings 52 are applied to the ceramic rods 46, 48, 50 coextensive with the last several turns of helices 42, 44 farthest from the electron gun 12 to provide a termination for waves reflected from the output circuit of the tube. Resistive coatings 52 may, for example, be provided by an aqueous suspension of graphite generally known as Aquadag. Helices 42, 44 commence and end at diametrically opposite points of the bifilar helix 16 and are maintained at ground potential during operation of the device. This may be accomplished by connections from helices 42, 44 through resistors 54, 56 respectively, to ground. Also, the collector electrode 14 is maintained at a potential of the order of 200 volts positive with respect to ground by means of a connection therefrom to the positive terminal of a battery 58 which is, in turn, referenced to ground.

As previously stated, the transition device 18 matches the impedance of the bifilar helix 16 to the output waveguide section 20. Transition device 18 comprises matching ferrules 60, 62 which are composed of non-magnetic material and have a length equal to several turns of the bifilar helix 16 which may, for example, be of the order of a free space wavelength at the mid-operating frequency of the device. The inside diameter of ferrules 60, 62 is of sufficient magnitude to enable the bifilar helix 16 to propagate a backward wave through the ferrules 60, 62 disposed concentrically about the helix 16 with little or no disturbance, i. e., without additional attenuation or reflection of the wave. Normally this would require that the inside diameter of the ferrules 60, 62 be of the order of 0.25 inch greater than the outside diameter of the bifilar helix 16. Structurally, the diameter of ceramic rods 46, 48, 50 are made equal to one-one half of this difference, i. e., 0.125 inch, so as to function as spacers for the helix when a ferrule is slipped over the outer periphery, as shown in Fig. 2.

The ferrules 60, 62 are disposed concentrically about the longitudinal axis of the bifilar helix 16 with their adjacent ends spaced equidistant from the extremity thereof nearest electron gun 12 thus leaving a gap be tween the femrrules 60, 62, as shown in Fig. 1. This gap may vary from 0.01 to 0.15 a free space wavelength at the mid-frequency of operation and is preferably of the order of 0.1 free space wavelength. Ferrules 60, 62 are connected. respectively, to helices 42, 44 at diametrically opposite points of the bifilar helix 16 at the extremity thereof nearest electron gun 12. The output waveguide section is disposed symmetrically about the gap between ferrules 60 and 62 in a manner such that the electric field of its dominant transverse electric modeis parallel to the longitudinal axis of the helix 16.

An adjustable shorting member 64 is provided, as. shown in Figs. 1 and 2, to permit the distance from the shorted termination 65 of waveguide section 20 to the gap between ferrules 60 and 62 to be adjusted for maximum power transfer. Also, in order to prevent energy leakage from the waveguide, sleeves 66 and 68 extend from the apertures in the sides of the waveguide coutiguously along the glass envelope 10, as shown in the figures. The coextensive distance between ferrule 62 and sleeve 66, and ferrule 60 and sleeve 68 is adjusted to produce a virtual shorting plane at the inner surface of the waveguide section 20. This distance would depend on the radius of the apertures in the waveguide and the relative dielectric constant of the glass envelope 10 and would normally be of the order of /h of the free-space wavelength corresponding to the frequency of operation of the device.

In its operation, the electron stream from the electron gun 12 would generate a backward wave on the bifilar helix 16. At diametrically opposite points along the helix 16, this wave is out-of-phase. Thus the match ing ferrules 60 and 62 are energized with electric fields that are 180 out-of-phase. These electric fields appear across the gap between ferrules 60, 62 and, in turn, launch a wave in the output waveguide section 20. The comparatively large diameter of the ferrules 60, 62 contributes towards the broad-band characteristics of the impedance match thus provided between the bifilar helix 16 and the waveguide section 20.

What is claimed is:

l. A backward wave oscillator comprising means for producing an electron stream, means for directing said electron stream along a predetermined path, a bifilar helix disposed contiguously along said predetermined path, whereby said electron stream generates a backwardwave on said helix, first and second matching ferrules having an inside diameter greater than the diameter of said helix disposed concentrically about the longitudinal axis of said helix with the adjacent ends thereof on each side of the extremity of said bifilar helix first entered by said electron stream, said first and second matching ferrules being connected, respectively, to diametrically opposite points of said bifilar helix at said extremity, and a waveguide output structure disposed symmetrically about said first and second matching ferrules, whereby said ferrules provide a transition between said bifilar helix and said waveguide output structure.

2. The backward wave oscillator as defined in claim 1 including additional means for providing a shorting plane transversely across said waveguide at a distance equal to guide wavelengths at the frequency of said backward wave from the longitudinal axis of said bifilar helix, wherein n is a positive integer.

3. A backward wave oscillator comprising a bifilar helix including first and second helices commencing at points on the circumference of said bifilar helix diametrically opposite each other; means for producing a hollow cylindrical electron stream having a diameter lessthan the inside diameter of said bifilar helix; means for directing said electron stream concentrically through said bifilar helix to generate a backward wave on said helix; first and second matching ferrules having an inside diameter greater than the diameter of said bifilar helix disposed concentrically about the longitudinal axis thereof, the adjacent ends of said ferrules being disposed on opposite sides of the commencement of said bifilar helix and con nected, respectively, to said diametrically opposite points thereof; and a waveguide output structure disposed symmetrically about said first and second matching ferrules, whereby said ferrules provide a transition for said backward wave from said bifilar helix to said waveguide output structure.

4. The backward wave oscillator as defined in claim 3 wherein the inside diameter of said first and second ferrules is at least 1.1 times the outside diameter of said bifilar helix 5. A transition structure between a bifilar helix and a rectangular waveguide comprising: a bifilar helix capable of propogating an electromagnetic wave, said bifilar helix including first and second helices commencing at points on the circumference thereof diametrically opposite each other; first and second matching ferniles having an inside diameter greater than the diameter of said bifilar helix disposed concentrically about the longitudinal axis thereof, the adjacent ends of said ferrules being disposed on opposite sides of the commencement of said bifilar helix and connected, respectively, to said diametrically opposite points thereof; a rectangular waveguide disposed symmetrically about the gap between the adjacent ends of said matching ferrules, the longitudinal axis of said bifilar helix being normal to the broad sides of said waveguide; and means for providing a transverse shorting plane across said waveguide at a distance equal to guide wavelength at the frequency of said wave, wherein n is a positive integer.

6. The transition structure as defined in claim 5 wherein the dimension of said gap between the adjacent ends of said matching ferrules is from 0.01 to 0.15 a free space wavelength at the mid-frequency of operation of the device.

References Cited in the file of this patent UNITED STATES PATENTS 

